[API Proposal]: `NullReferenceException.ThrowIfNull`

[MichalPetryka]

Background and motivation

Some code needs to throw NullReferenceException instead of ArgumentNullException, either for performance or compatibility reasons.

Currently the code relies on using weird patterns to trigger that, which is both not nice to write and obscures the intent of code, introducing need for comments.

Introducing an explicit way to trigger a nullcheck would make the code clearer to any reader.

The check would be implemented using a dereference by runtime, so invalid pointers would cause an AccessViolationException.

API Proposal

namespace System;

public class NullReferenceException
{
    public static void ThrowIfNull(object i);
    public static void ThrowIfNull(void* ptr);
    public static void ThrowIfNull<T>(ref readonly T r);
}

API Usage

NullReferenceException.ThrowIfNull(obj);
NullReferenceException.ThrowIfNull(ptr);
NullReferenceException.ThrowIfNull(ref r);

Alternative Designs

People can continue doing the obscure current variants:

_ = obj.field; // needs to know the type
_ = Unsafe.ReadUnaligned<byte>(ptr); // Null pointers are UB in C#.
_ = Unsafe.ReadUnaligned<byte>(ref Unsafe.As<T, byte>(ref r)); // Roslyn optimizes out unused derefs.

Risks

No response

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-runtime
See info in area-owners.md if you want to be subscribed.

[hamarb123]

My main question here is whether it should be checking null, or just that the byref can't be read. Imo, with the current name it should check for just null, but I presume this would be more expensive than just a simple read of the byref.

Especially with pointers it would definitely have to check null ofc, so why should byrefs be different; but if they are, it becomes unnecessary perf lost for 99.99% of cases, since most people aren't making byrefs to non-readable pages for example.

Maybe the language should finally just treat null byrefs not as UB & allow writing _ = myByref; to do the optimal de-reference check? Currently it optimizes that away.

[jkotas]

Some code needs to throw NullReferenceException instead of ArgumentNullException, either for performance or compatibility reasons.

Code like this is very rare. I do not see a problem with the few places that need this to use the existing alternatives.

[tannergooding]

I also agree that this is expected to be exceptionally rare and not worth exposing. You can always provide it as your own static extension member if you really want to use that exact signature.